relationship of polar unit vectors to rectangularOK, this is a good clue. I'm getting the picture that if I move forward and learn about transformations between the coordinate systems (as Brady mentions above) I may be able to come back at that time and understand this. So thank you guys.

May15

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relationship of polar unit vectors to rectangularThanks for the pointers to related concepts - I will look further into this. In context, the author is contrasting the Cartesian unit vectors $\hat{i}$ and $\hat{j}$ with the 2D Polar unit vectors $\hat{r}$ (radius) and $\hat{\theta}$ (degrees from x-axis). Immediately before the 2 equations he says "An important consequence [of the definition of polar unit vectors] is that the directions of $\hat{r}$ and $\hat{\theta}$ will be different at different locations...the dependence of the polar unit vectors on position can be seen in the following relations:" Thanks again for your help.